add compat for pytest 3.7 and tox config for (some of) the versions i could still get working

pytest_stepwise/compat.py

@@ -5,6 +5,12 @@ try:
 except ImportError:
     from pytest_cache import Cache
 
+try:
+    # pytest 3.7+
+    Cache = Cache.for_config
+except AttributeError:
+    pass
+
 
 if hasattr(pytest, 'hookimpl'):
     tryfirst = pytest.hookimpl(tryfirst=True)

tests/test_pytest_stepwise.py

@@ -132,4 +132,7 @@ def test_stop_on_collection_errors(broken_testdir):
     result = broken_testdir.runpytest('-v', '--strict', '--stepwise', 'working_testfile.py', 'broken_testfile.py')
 
     stdout = result.stdout.str()
+    if pytest.__version__ < '3.0.0':
         assert 'Error when collecting test' in stdout
+    else:
+        assert 'errors during collection' in stdout